In order to realize massive high-speed ultra-long distance optical data communication, a coherent optical communication system based on optical phase modulation/demodulation has become one of mainstream solutions. At a transmitting terminal of the coherent optical communication system, a quadrature phase shift keying (QPSK) electro-optic modulator based on parallel Mach-Zehnder interferometer (MZI) structure has been widely used. A single MZI optical structure divides an input light into two paths of light which combine after being transmitted for a certain distance, and interference occurs. By changing the phase difference of the two paths of transmitting light from MZI through an applied voltage, the intensity and phase of an output light may be changed, and the MZI structure was widely favored in the field of electro-optic modulation devices due to this characteristic. A single MZI structure was biased to a non-light bias point where the output light intensity is the lowest, and then applied bias voltages with opposite polarities on its two arms, and the polarities of the bias voltages are periodically changed, so that a periodic optical signal with constant light intensity and a phase difference of 180° may be obtained and binary phase modulation of light (BPSK) may be formed. After the two MZIs are connected in parallel, the two paths of BPSK signals are orthogonally combined in a phase difference of 90° to obtain a QPSK signal. In 100G coherent optical communication, an in-phase and quadrature-phase (IQ) electro-optic modulator based on a parallel MZ structure is written into an international standard as a standard solution.
Before the parallel MZ electro-optic modulator is used, it is necessary to find an optimum operating point voltage of its bias voltage in order to make the modulator operate be in an optimum state. Fujitsu Company, a supplier of lithium niobate modulator, provides a debugging method for an operating point voltage of the lithium niobate modulator in its product specification, but this method is not suitable for debugging an operating point voltage of modulators made of all materials, such as a silicon-based electro-optic modulator biased by the thermo-optical effect. Due to the fact that the silicon optical modulator is biased by the thermo-optical effect, after a radio frequency (RF) signal is input, the RF energy loss generates heat, which makes the voltage of the operating point voltage of the modulator change, and it is difficult to adopt a method for finding the operating point voltage by a general lithium niobate modulator to debug the operating point voltage of the silicon optical modulator.